Heretofore the most widely used practice for sending voice communications with a hard limited transmitter was with the use of binary phase shift keying (BPSK), quadraphase shift keying (QPSK) modulation, or frequency modulation (FM). In order to minimize the spectrum occupancy of the transmitted signal a constant carrier envelope is required. Alternative techniques using amplitude modulation require linear transmitters since they do not have a constant envelope.
Where two such communication signals are received at a site for communication conferencing they are assigned to a common demodulation channel. When this occurs, the demodulated signals will contain components from the two communication signals, the resultant signal being modulated with the respective incoming signals. If the modulation is, for example, pulse duration modulation (PDM) then the recovered intermediate frequency (IF) signal is in effect, double sideband suppressed carrier modulated in a PDM fashion. Coherent recovery of this signal requires pre-injection of a phase coherent carrier signal to mix double sideband suppressed carrier signals to baseband. Inasmuch as there are two signals whose carriers are typically not the same frequency, there cannot be simultaneous demodulation of the two signals coherently to baseband. One or both of the incoming signals will therefore be highly distorted by this process. This problem can partially be solved by using non-coherent demodulation but this still leaves the problem of carrier amplitude modulation. Even though a double sideband suppressed carrier modulation signal has no average carrier, a carrier does exist instantaneously. Within a conference call the carriers will at times be equal and opposite in phase. They will cancel for that portion of time that the PDM signals are in the same state. This can occur during quiet periods to cause total carrier loss for significant periods of time. Since the carriers are not likely to have precisely the same frequency, they will alternately cancel and add in such a manner that the resulting demodulated audio signal and noise will be amplitude modulated at the rate of the difference of the carrier frequencies.
In view of the above problems there exists a need for a phase modulation system which has a constant carrier envelope for transmission through non-linear transmitter stages and which likewise can be demodulated non-coherently in a conventional single sideband manner without carrier beat.